<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        var plusOne = function (digits) {
            for(let i = digits.length-1;i>=0;i--){
                if(digits[i]!=9){
                    digits[i]++
                    return digits;
                }
                else{
                    digits[i] = 0;
                }
            }
            digits.unshift(1)
            return digits
        }
        var digits = [8,9]
        console.log(plusOne(digits))
    </script>
</body>

</html>